A borehole is typically drilled using a drill bit which is attached to an end of a drill string. Rotary drilling is performed by rotating the drill bit. The drill bit may be rotated by rotating the drill string, by rotating the drill bit with a downhole drilling motor, or in some other manner.
A roller cone drill bit includes cones which rotate as the drill bit is rotated. Teeth which are positioned on the cones roll along the bottom of the borehole as the cones rotate. The teeth impact the bottom of the borehole as they roll and thereby crush and disintegrate rock in order to advance the borehole.
A fixed cutter drill bit typically includes no moving parts, but includes cutters which are attached to the body of the drill bit and which rotate with the drill bit as the drill bit is rotated. The cutters scrape the borehole as the drill bit rotates, thereby shearing rock in order to advance the borehole.
A cutter on a fixed cutter drill bit is typically comprised of a cutter element, such as an “abrasive” or “superabrasive” cutter element, which performs the shearing action. An abrasive cutter element may be comprised of tungsten carbide, another carbide material, ceramic and/or some other material. A superabrasive cutter element may be comprised of natural diamond, a synthetic diamond material such as polycrystalline diamond compact (PDC) or thermally stable diamond (TSP), or may be comprised of some other material such as cubic boron compact or diamond grit impregnated substances.
A cutter on a fixed cutter drill bit may be further comprised of a substrate to which the cutter element may be affixed. For example, a PDC or TSP cutter element may be comprised of a “diamond table” which may be affixed to a substrate such as tungsten carbide in order to provide the complete cutter. The cutter element defines a cutting face which contacts the borehole in order to perform the shearing action. The perimeter of the cutting face defines an edge of the cutting face. The cutting face may be flat or may be contoured. For example, a contoured cutting face may be comprised of a raised shape.
A PDC or TSP cutter element may typically be affixed to a substrate by applying high temperature and high pressure to the cutter element and substrate in the presence of a catalyst so that the materials of the cutter element and the substrate bond with each other.
Fixed cutter drill bits are therefore typically comprised of a drill bit body and a plurality of cutters which are attached to the drill bit body. The drill bit body is typically constructed of steel or of a matrix containing an erosion resistant material such as tungsten carbide. The cutters are typically attached to the drill bit body by an adhesive or by brazing. The cutters may be received in cutter pockets in the drill bit body in order to facilitate the attachment of the cutters to the drill bit body.
The drill bit body and the cutters are configured to provide an overall design for the drill bit, having regard to considerations such as drilling performance, durability and hydraulic performance of the drill bit.
As one example, the drill bit body typically includes a plurality of blades to which the cutters are attached and between which fluids and cuttings may pass. Because the cutters are typically attached to the blades of the drill bit, increasing the number of blades on a fixed cutter drill hit will generally increase the number of cutters which may be attached to the drill bit body, thereby increasing the “cutter count” and the “cutter density” on the drill bit.
Generally, the drilling performance (i.e. rate of penetration) which can be achieved by a fixed cutter drill bit is inversely proportional to the number of blades and cutters which are included in the drill bit. In other words, the greater the number of blades and the greater the number of cutters, the lower the rate of penetration which may be expected from the drill bit.
Generally, the durability of the drill bit is proportional to the number of blades and cutters which are included in the drill bit. In other words, the greater the number of blades and the greater the number of cutters, the longer the drill bit may be expected to function without experiencing excessive wear.
Generally, the hydraulic performance of the drill bit is inversely proportional to the number of blades which are included in the drill bit. In other words, the greater the number of blades, the less area which is available between the blades for the passage of fluids and cuttings, and the more resistance which is provided to the passage of fluids and cuttings past the drill bit.
As a second example, drill bits may embody different cutter shapes, cutter geometries and cutter layouts. These cutter shapes, cutter geometries and cutter layouts may be directed at maximizing drilling performance (i.e., aggressiveness) or durability of the drill bit.
Generally, angular cutters (in which the perimeter of the cutting face is generally polygonal, with straight sides connected by angles) are relatively aggressive, but are susceptible to failure. The shape of the perimeter of the cutting face of an angular cutter may, for example, be generally triangular, square, pentagonal, etc.
Generally, curved cutters (in which the perimeter of the cutting face is generally curved or circular, with a continuous curved or circular shape) are relatively durable, but tend to be less aggressive than angular cutters. The shape of the perimeter of the cutting face of a curved cutter may, for example, be generally round, oval, etc.
Generally, chamfering the edges of the perimeter of an angular cutter or a curved cutter can increase the durability of the cutter, while sacrificing some aggressiveness of the cutter.
In abrasive types of rock and/or in formations which subject the cutters to a high frequency of “impact” or transitional drilling, angular cutters may tend to break more frequently and more catastrophically than curved cutters.
Curved cutters may, however, encounter more difficulty than angular cutters in shearing through highly plastic formations. To reduce this difficulty, reduced chamfering of the edge of the cutting face can be provided to a curved cutter to improve the drilling performance of the bit, but at the expense of decreased durability, since the sharper cutter edge may have a greater tendency to chip or break.
All of these and many other environments may be encountered in the drilling of a single borehole. As a result, the drilling performance and durability of a fixed cutter bit may represent a compromise between design considerations such as the shape of the perimeter of the cutting face and or the extent and geometry of the chamfering of the edge of the cutting face.
Another potential problem inherent in fixed cutter bits relates to the cutters which are located at the distal end of the drill bit, and in particular, such cutters which are located at or adjacent to the centerline of the drill bit. These “center” cutters are susceptible to cutter damage due to “out of center rotation”, which can occur when the centerline of the drill bit does not coincide with the centerline of the borehole being drilled. In such circumstances, a cutter located at or adjacent to the centerline of the drill bit may cross the centerline of the borehole during drilling, causing the cutter to move backward against the material being sheared and potentially resulting in damage to the cutter due to improper backward loading.
One solution to this potential problem is to avoid locating cutters at or directly adjacent to the centerline of the drill bit, thereby leaving a “core” of purposely uncut hole bottom at the end of the borehole and minimizing the risk of cutters crossing the centerline of the borehole during drilling. The presence of this core can be both beneficial and detrimental. The principal benefit of the core is that it can enhance drill bit stability, with the potential enhanced stability being generally proportional to the size of the core. The principal detriment of the core is that the core can cause damage to the drill bit and the cutters adjacent to the core because of undesirable side loading which may be imposed on the adjacent cutters as they contact the core during drilling.
There remains a need for fixed cutter drill bits which facilitate reasonable compromises with respect to the drilling performance, durability, and stability which can be achieved with the drill bit.